Swamp Thing can be seen below Harbinger (the floating blonde in the upper left.)

Swamp Thing made a mandatory cameo appearance in DC Comics' CRISIS ON INFINITE EARTHS maxi-series (issue #5) in 1985, along with characters in every other DC series. (Art by George Perez, written by Marv Wolfman.) To the credit of writer Alan Moore, issue #46 blended in with the company-wide plot intervention fairly well.